The present invention relates to an electrophotographic image forming method, an electrophotographic image forming apparatus and an electrophotographic image forming process unit. More specifically, it relates to an electrophotographic image forming method using a magnetic single component developer.
In recent years, dry development in an electrostatic copying method has found wide acceptance in the field of electrophotographic apparatus for personal users, such as printers and facsimiles, and further downsizing and weight reduction of apparatus have been in demand. Besides, in these electrophotographic apparatus, the speed-up of the electrophotographic process has been required, and various improvements or new developments of the image forming method have been studied for providing a higher-image quality under severe conditions.
The dry development in various electrostatic copying methods which have been currently put to practical use includes two-component developer using a toner and a carrier such as an iron powder and single component development in the absence of a carrier. The use of two-component development has been most widespread. This development involves, however, the following defects. That is, a developer is deteriorated by adhering it to the surface of the carrier, or the developer alone is consumed. Consequently, the mixing ratio of the toner and the carrier has to be kept constant lest the amount of the developer is decreased. A developing unit has therefore a large size.
Meanwhile, single component development is free from these defects because of the absence of the carrier, and is advantageous in downsizing the apparatus. For this reason, single component development is becoming the preferred method of development in electrophotographic apparatus for small offices or personal users. Single component development is roughly classified into nonmagnetic single component development using a nonmagnetic toner and magnetic single component development using a magnetic toner. Since a magnetic material is not used in the toner in the nonmagnetic single component development, it is suited for coloration. However, the developer is carried on a developer bearing member mainly by an electrostatic force alone by mutual triboelectric charging of the developer and the developer bearing member. Accordingly, when a charge amount of the developer is small, problems such as fogging of a non-image area and contamination of an apparatus tend to occur. Thus, this development is not preferred in a monochromic electrostatic copying method. Meanwhile, in magnetic single component development, an image is developed while carrying a magnetic toner with a developer bearing member having a magnetic field generation unit such as a magnet thereinside. Therefore, this development is free from the problems associated with the nonmagnetic single component development. Nevertheless, this development suffers from a problem that scattering of toner tends to occur when it is applied to a high-speed machine.
The invention has been made in view of these circumstances, and provides an electrophotographic image forming method, an electrophotographic image forming apparatus and an electrophotographic image forming process unit.
According to an aspect of the invention, the electrophotographic image forming method includes a charging step of charging an electrostatic latent image bearing member, an exposure step of exposing the electrostatic latent image bearing member to exposure light to form an electrostatic latent image, a developing step of supplying a developer from a developer bearing member to the electrostatic latent image bearing member to develop the electrostatic latent image and form a toner image, and a transfer step of transferring the toner image on a transfer member. In this method, a process speed of the electrostatic latent image bearing member is 200 to 500 mm/sec, an outer diameter of the electrostatic latent image bearing member is 15 to 35 mm, an outer diameter of the developer bearing member is 10 to 25 mm, and a magnetic single component developer of which the residual magnetization in a measured magnetic field of 796 kA/m is 0.5 to 5.0 Am2/kg is used as the developer.
According to another aspect of the invention, the electrophotographic image forming apparatus includes a charging unit that charges an electrostatic latent image bearing member, an exposure unit that exposes the electrostatic latent image bearing member to exposure light to form an electrostatic latent image, a developing unit that supplies a developer from a developer bearing member to the electrostatic latent image bearing member to develop the electrostatic latent image and form a toner image, and a transfer unit that transfers the toner image on a transfer member. In the apparatus, an outer diameter of the electrostatic latent image bearing member is 15 to 35 mm, an outer diameter of the developer bearing member is 10 to 25 mm, a magnetic single component developer of which the residual magnetization in a measured magnetic field of 796 kA/m is 0.5 to 5.0 Am2/kg is used as the developer and a process speed of the electrostatic latent image bearing member is 200 to 500 mm/sec.
According to another aspect of the invention, the electrophotographic image forming process unit includes an electrostatic latent image bearing member, a toner chamber that stores a developer and a developing unit that supplies a developer from a developer bearing member to the electrostatic latent image bearing member to develop the electrostatic latent image and form a toner image, in which an outer diameter of the electrostatic latent image bearing member is 15 to 35 mm, an outer diameter of the developer bearing member is 10 to 25 mm, a magnetic single component developer of which the residual magnetization in a measured magnetic field of 796 kA/m is 0.5 to 5.0 Am2/kg is used as the developer and a process speed of the electrostatic latent image bearing member is 200 to 500 mm/sec.
According to the electrophotographic image forming method and the electrophotographic image forming apparatus of the invention, the process speed and the outer diameter of the electrostatic latent image bearing member and the outer diameter of the developer bearing member are controlled to the foregoing specific ranges, whereby the electrostatic latent image bearing member and the developer bearing member are downsized and the electrophotographic process is sped up without excessively increasing a centrifugal force acting on the toner on the latent image bearing member. Further, the magnetic single component developer to meet the specific condition of the residual magnetization is used to appropriately decrease the magnetic cohesive force of the toner on the electrostatic latent image bearing member, whereby a phenomenon of adhering an excessive amount of the toner to the electrostatic latent image bearing member is prevented satisfactorily. Accordingly, the downsizing and the weight reduction of the electrophotographic apparatus and the speed-up of the electrophotographic process can be attained at the same time, and good image quality can be obtained without scattering of toner.